1. Field of the Invention
The present disclosure relates to image heating apparatuses configured to heat images formed on recording materials and to heaters used in the image heating apparatuses.
2. Description of the Related Art
Image heating apparatuses are provided in image forming apparatuses such as a copying machine and a printer to serve as fixing apparatuses. An image heating apparatus that includes an endless belt, a ceramic heater, which makes contact with an inner surface of the endless belt, and a pressure roller, which, along with the ceramic heater, forms a fixing nip portion with the endless belt provided therebetween, is one of such image heating apparatuses. Continuous printing on small-sized sheets with an image forming apparatus that includes such an image heating apparatus causes the temperature of an area in a lengthwise direction of the fixing nip portion where the sheets do not pass through to gradually rise (i.e., non-sheet-passing part temperature rise). An excessive rise in the temperature of a non-sheet-passing part may cause damage to parts in an apparatus, or printing on a large-sized sheet with the temperature of the non-sheet-passing part remaining high may cause toner on the area corresponding to the non-sheet-passing part of the small-sized sheets to be overheated and be offset onto the belt (i.e., high temperature offset).
Japanese Patent Application Laid-Open No. 2003-317898 and Japanese Patent Application Laid-Open No. 2003-007435 discuss a method of providing a thermally conductive anisotropic layer such as graphite on a ceramic heater to suppress the non-sheet-passing part temperature rise. Graphite has a layered structure of hexagonal plate crystal formed of carbon, and the layers are bonded by the van der Waals force. Graphite has higher thermal conductivity in a direction parallel to the surface of the ceramic heater (i.e., direction parallel to the plane of a covalently bonded layer in graphite). Thus, providing graphite on a ceramic substrate enables the rise in the temperature of a non-sheet-passing part of small-sized sheets to be suppressed.
Furthermore, graphite has low thermal conductivity in the thickness direction thereof (i.e., direction perpendicular to the plane of the covalently bonded layer in graphite). Thus, heat dissipation to a holder supporting the ceramic heater can be suppressed, and heat can be efficiently provided to paper.
Bringing a temperature detection member into contact with a ceramic heater to detect the temperature of the ceramic heater is a generally used method. Graphite, however, has low thermal conductivity in the thickness direction thereof. Thus, when the temperature of the ceramic heater is detected with a thermally conductive anisotropic layer such as graphite provided therebetween, there is a delay in response of the temperature detection member.